Spider bites are usually harmless. In fact, many bites attributed to spiders turn out to have been inflicted by other bugs. Skin infections also have been mistaken for spider bites.

Only a few types of spiders have fangs long enough to penetrate human skin and venom strong enough to severely affect a human being. In the U.S., these include the black widow spider and the brown recluse spider.

Black widow spider bites can cause severe abdominal pain and cramping, while brown recluse spider bites can cause the skin around the bite to die. Both these spiders generally live in undisturbed areas, such as attics or sheds, and don't bite unless threatened.

Typically, a spider bite looks like any other bug bite — a red, inflamed, sometimes itchy or painful bump on your skin — and may even go unnoticed. Harmless spider bites usually don't produce other symptoms.

Black widow spider bites

Signs and symptoms of a black widow spider bite may include:

Pain. Typically beginning within an hour of being bitten, pain can spread from the bite site into your abdomen, back or chest.

Cramping. Abdominal cramping or rigidity can be so severe that it's sometimes mistaken for appendicitis or a ruptured appendix.

Sweating. Excessive sweating can occur around the bite mark or may involve the entire limb.

Brown recluse spider bite

The pain associated with a brown recluse spider bite typically increases during the first eight hours after the bite. The bite usually heals on its own in about a week. In a minority of cases, the skin at the center of the bite can become dusky red and then evolve into a deep open sore (ulcer) that enlarges as the surrounding skin dies. The ulcer usually stops growing within 10 days after the bite, but full healing can take months.

When to see a doctor

Seek prompt medical assistance if you believe you've been bitten by a spider and you're experiencing:

Severe pain

Abdominal cramping

A growing ulcer at the bite site

Severe spider bite symptoms occur as a result of injected spider venom. The severity of symptoms depends on the type of spider, the amount of venom injected and how sensitive your body is to the venom.

Although dangerous spider bites are rare, your risk of being bitten increases if you live in the same areas that the spiders do and you happen to disturb their habitat. Both black widow and brown recluse spiders prefer warm climates and dark, dry places.

Black widow habitat

Black widow spiders can be found throughout the U.S. but more so in the Southwestern states. They prefer to live in:

Sheds

Garages

Unused pots and gardening equipment

Woodpiles

Brown recluse habitat

Brown recluse spiders are found most commonly in the southern Midwest and in limited areas of the South. Recluses are so named because they like to hide away in undisturbed areas. They mostly prefer to live indoors, in places such as:

The clutter of basements or attics

Behind bookshelves and dressers

In rarely used cupboards

Outside, they seek out dark, quiet spots, such as under rocks or in tree stumps.

Very rarely, a bite from a black widow or brown recluse spider may be deadly, particularly in children and in older people with serious health problems.

If you've been bitten by a spider that you suspect is a black widow or brown recluse, call your primary care doctor or go to an urgent care center. If your doctor has online services, an option may be to email a picture of the spider to your doctor.

What you can do

To help your doctor understand your symptoms and how they might relate to a spider bite, you can:

Bring the spider or a photo of the spider with you

Write down any symptoms you're experiencing

Write down questions to ask your doctor

Some basic questions you might want to ask include:

If this is a dangerous spider bite, what's the next best step?

If this isn't a spider bite, what are possible causes for my symptoms or condition?

Do I need any tests?

How long will my symptoms last?

What is the best course of action?

What to expect from your doctor

Your doctor is likely to ask you a number of questions. Being ready to answer them may reserve time to go over any points you want to spend more time on. Your doctor may ask:

When did you begin experiencing symptoms?

What were you doing in the hours before your symptoms started?

Have your symptoms gotten worse?

Does anything relieve your symptoms or make them worse?

Your doctor may suspect a spider bite based on your history and your signs and symptoms, but a specific diagnosis is difficult to confirm. Confirmation requires:

An eye witness to the bite

Identification of the spider by an expert

Exclusion of other possible causes

Black widow identification

Some clues for identifying black widow spiders include:

Shiny black body

Large round abdomen

Red hourglass on underside of abdominal area

Central body about a half-inch (12 to 13 millimeters) long

Entire body, including legs, can be more than an inch (2.5 centimeters) across

Brown recluse identification

Some clues for identifying brown recluse spiders include:

Color ranges from yellowish tan to dark brown

Dark violin shape on top of the leg attachment segment

Six eyes — a pair in front and a pair on both sides — rather than the usual spider pattern of eight eyes in two rows of four

Central body is a quarter-inch to three-quarters of an inch (6 to 19 mm) long

Entire body, including legs, can be more than an inch (25 mm) across

For most people with spider bites, including black widow and brown recluse spider bites, the following treatment measures are all that's required:

Clean the bite with mild soap and water.

Apply cold packs to the bite, to reduce pain and inflammation.

If the bite is on an extremity such as an arm or leg, keep it elevated.

Take over-the-counter pain relievers as needed.

Observe the bite for signs of infection.

Your doctor may also recommend a tetanus booster shot if you haven't had one in the last five years. Antibiotics may be prescribed if the bite becomes infected.

Black widow anti-venom

If a black widow bite is causing intractable pain or life-threatening symptoms, your doctor may recommend an anti-venom, which may be injected into a thigh muscle or given through a vein (intravenously). Anti-venom can cause serious allergic reactions, so it must be used with caution.

Spiders in general, including the black widow and brown recluse, bite only in defense, when being crushed between your skin and another object.

To prevent spider bites:

Wear a long-sleeve shirt, hat, gloves and boots when handling stored boxes or firewood, and when cleaning out sheds, garages, basements, attics and crawl spaces.

Inspect and shake out gardening gloves, boots and clothing that have been unused for a while.

Use insect repellents, such as DEET or Picaridin, on clothing and footwear.

Keep insects and spiders out of the house by installing tight-fitting screens on windows and doors, and caulking or sealing cracks or crevices where spiders can come in.

Discard old boxes, clothing and other unwanted items from storage areas.

Store items you want to keep off of the floor and away from walls.

Remove piles of rocks or lumber from the area around your house.

Avoid storing firewood against the house.

Vacuum spiders and spider webs and dispose of them in a sealed bag outside to prevent re-entry into the house.

References

Vetter RS, et al. Approach to the patient with a suspected spider bite: An overview. http://www.uptodate.com/home. Accessed March 12, 2013.

Reprint Permissions

A single copy of these materials may be reprinted for noncommercial personal use only. "Mayo," "Mayo Clinic," "MayoClinic.org," "Mayo Clinic Healthy Living," and the triple-shield Mayo Clinic logo are trademarks of Mayo Foundation for Medical Education and Research.